ONLINE PORTAL IMAGING - IMAGE QUALITY DEFINING PARAMETERS FOR PELVIC FIELDS - A CLINICAL-EVALUATION

Purpose: A test of several image enhancement techniques, performed on on-line portal images in real clinical circumstances, is presented. In addition a score system enabling us to evaluate image quality on pelv ic fields is proposed and validated. Methods and Materials: Localizati on images (n = 546) generated by an on-line portal imaging system duri ng the treatment of 13 patients on pelvic fields were obtained by deli vering a radiation dose of 6-8 cGy by an 18 MV photon beam, and record ed with a silicon intensified target video camera with adjustable gain , kV- and black level. Set-up errors were corrected before continuing irradiation. A scoring system based on the number of visible bone-soft tissue edges and transformed to a scale 0 to 5 was developed to judge image quality. A validation of this classification of images was perf ormed with the use of transsectional bone-densities (bone-density rad iological path length) specified at the score defining landmarks. A hi gh pass filter was used on all images, additional online open field su btraction was performed on 242 fields. Off-line study was performed in which a panel consisting of two groups (one composed of three radiati on oncologists, the other of three radiotherapy technologists), scored 470 pelvic fields without further enhancement, and the same images wi th Contrast Limited Adaptive Histogram Equalization (CLAHE) (Pizer et al.). Two different clipping levels (3.0 and 5.0) were studied. Result s: Gender and transsectional bone-densities were the most defining pat ient-related factors influencing image quality. Camera settings, gantr y angle, and image post-processing were important non-patient-related factors. All investigators judged CLAHE to ameliorate low contrast ima ges and to deteriorate good quality images (p < 0.001).